With a recent advent of large-sized LCDs and a gradual expansion of their use from portable devices, such as mobile phones, lap-top computers, etc., to home appliances, such as wall mounted flat panel TVs, there is a demand for LCDs with high image quality, high definition and wide viewing angle. In particular, TFT-driven thin film transistor LCDs (TFT-LCDs) of which the individual pixels are independently driven are much superior in response time of liquid crystals, realizing high-quality motion pictures, and thus increasingly used in a wider range of applications.
To be used as an optical switch in the TFT-LCDs, liquid crystals are required to initially align in a defined direction on a layer including innermost TFT of the display cell. For this, a liquid crystal alignment layer is used. For the liquid crystal alignment to occur, a rubbing process has been chiefly adopted. In the rubbing process, a rubbing cloth with fine projections is moved in a defined direction along the surface of the liquid crystal alignment layer, leaving grooves in a defined direction on the surface of the liquid crystal alignment layer. Thus, liquid crystals can be aligning along the grooves.
However, the rubbing process has two problems: (1) it possibly produces static electricity between the rubbing cloth and the thin film transistor (TFT) or the color filter (CF) substrate to cause damages on the TFT; and (2) fine fibers are, in many cases, released from the rubbing cloth to lead to defectives with foreign substances, which is an obstacle to the enhancement of production yield.
To overcome these problems, there has been widely suggested a new approach using a photoalignment method to align a liquid crystal alignment layer upon exposure to a light such as UV radiation. For the photoalignment to occur, an alignment layer including a photoalignment (photosensitive) polymer is formed on the bottom of a liquid crystal layer and exposed to linearly polarized UV radiation to cause a photoreaction. As a result, photoalignment takes place to align the main chain of the photoalignment polymer in a defined direction. And, the liquid crystals contained in the liquid crystal layer are aligned by the effect of the photoaligned alignment layer.
The representative example of the photoalignment is photopolymerization-based photoalignment as disclosed by M. Schadt et al. (Jpn. J. Appl. Phys., Vol 31., 1992, 2155), Dae S. Kang et al. (U.S. Pat. No. 5,464,669), and Yuriy Reznikov (Jpn. J. Appl. Phys. Vol. 34, 1995, L1000). The photoalignment polymers used in these patent and research papers are mostly polycinnamate-based polymers, including poly(vinylcinnamate) (PVCN) or poly(vinyl methoxycinnamate) (PVMC). When the polymers are subjected to photoalignment, the double bond of cinnamate under UV radiation participates in a [2+2] cycloaddition reaction to form cyclobutane, which renders anisotropy to align liquid crystal molecules in one direction, thereby inducing alignment of the liquid crystals.
However, there is a downside to such conventional photoalignment polymers: poor structural and thermal stabilities of the main chain and unsatisfactory alignment properties. The photoalignment polymers, typically dissolved in an organic solvent, are applied to a substrate to form an alignment layer or a liquid crystal alignment layer. But, the conventional photoalignment polymers have such a low solubility to organic solvents that there is the difficulty in forming an alignment layer using them.